Solution taking higher time on small contraints

1

I recently solved a question where my algo has complexity of O(N).

Surprisingly, on constraint where N<=1000, Solution takes 0.97 seconds ; but on constraint where N<=100000 ;
it takes 0.00 seconds (C++ Solution). Why is that happening...??? And even why at a constraint of N<=1000 ; solution is taking nearly 1 second time even in C++ ?

First operation just adds character B to the string whereas the second operation makes a copy of first string, adds character and then assigns to the original string. Therefore your code might look like O(n) but its not.